1. Field of the Invention:
The present invention relates to an acoustic surface wave device. More specifically, the present invention relates to an improvement in an acoustic surface wave device including a combination of a piezoelectric material substrate and an interdigital electrode formed thereon wherein the interdigital electrode is divided in the progagating direction of the acoustic surface wave into a plurality of electrode portions and these electrode portions are electrically connected in a series fashion.
2. Description of the Prior Art
As well known, the impedance of an acoustic surface wave device is determined by an electrostatic capacitance formed by an interdigital electrode and a piezoelectric material substrate and the electrostatic capacitance is further determined as a function of a capacitance value between the paired adjacent electrode fingers that are overlapped with each other in the interdigital manner and the number of such pairs. Accordingly, it would be appreciated that the impedance of the acoustic surface wave device is determinable depending on the number of pairs of the adjacent electrode fingers that are overlapped with each other in the interdigital manner. On the other hand, in actual application of an acoustic surface wave device, the impedance of the device must be matched with the impedance of another circuit to which the device is coupled. Of late, therefore, for the purpose of facilitating the impedance matching of an acoustic surface wave device with another circuit to which the device is coupled by reducing an electrostatic capacitance thereby to increase the impedance, the interdigital electrode is divided in the propagating direction of the acoustic surface wave into a plurality of electrode portions and the divided electrode portions are electrically connected in a series fashion. Such type of acoustic surface wave device is often referred to as a divided type acoustic surface wave device. An acoustic surface wave device comprising a divided interdigital electrode structure is disclosed for example in U.S. Pat. No. 3,600,710, issued Aug. 17, 1971 to Robert Adler and entitled "ACOUSTIC SURFACE WAVE FILTER"; and U.S. Pat. No. 3,675,054, issued July 4, 1972 to William Stanley Jones and entitled "SERIES CONNECTION OF INTERDIGITATED SURFACE WAVE TRANSDUCERS".
FIGS. 1 to 3 show several examples of elecrodes patterns of the conventional divided type acoustic surface wave devices. Referring to FIG. 1, the device shown comprises a transducer 100 including an interdigital electrode including two divided electrode portions 10 and 20. The divided electrode portion 10 comprises comb shaped electrodes 1 and 3 which are interdigitated with each other. Similarly, the divided electrode portion 20 comprises comb shaped electrodes 2 and 3 which are interdigitated with each other. The comb shaped electrode 3 in the divided electrode portion 10 is connected to the comb shaped electrode 3 in the divided electrode portion 20 through a connecting finger 4. If and when the comb shaped electrodes 1 and 2 are connected to different potentials, the comb shaped electrode 3 is placed in an intermediate potential of those of the comb shaped electrodes 1 and 2, i.e. a common potential with respect to the two divided electrode portions 10 and 20. The width of the electrode fingers of the respective comb shaped electrodes 1, 2 and 3 is selected to be 1/4.lambda., while the spacing between the adjacent electrode fingers is also selected to be 1/4.lambda., where .lambda. is the wave length of the acoustic surface wave propagating along the piezoelectric material substrate.
The electrode patterns of the acoustic surface wave devices shown in FIGS. 2 and 3 are different from that of the device shown in FIG. 1 in the manner of connection of two divided electrode portions 10 and 20. More specifically, the FIG. 1 device comprises the connecting electrode finger 4 in the dividing region which has the width of 3/4.lambda., whereby the interdigital electrodes of the two divided electrode portions 10 and 20 are electrically connected in a series fashion. On the other hand, the FIG. 2 device comprises two adjoining electrode fingers 5 and 6 which are placed in the common potential through connection to the common comb shaped electrode 3. The FIG. 3 device comprises in the dividing region an electrode finger 7 placed in the same potential as that of the comb shaped electrode 1 and an electrode finger 8 placed in the same potential as that of the comb shaped electrode 2 which are disposed to be adjacent to each other so as to be overlapped with each other.
FIGS. 4 and 5 show other examples of electrode patterns of the interdigital electrodes of the further conventional acoustic surface wave devices. FIGS. 4 and 5 are similar to FIGS. 2 and 3, respectively, but are different from FIGS. 2 and 3 in that each electrode finger of the devices shown in FIGS. 4 and 5 is formed in a split type although each finger of the devices shown in FIGS. 2 and 3 is a solid type. A split type electrode of the interdigital electrodes for an acoustic surface wave device is disclosed in U.S. Pat. No. 3,727,155 issued Apr. 10, 1973 to Adrian J. DeVries and entitled "ACOUSTIC SURFACE WAVE FILTER".
By dividing one interdigital transducer into a plurality of divided electrode portions and by connecting the divided electrode portions in a series fashion, an electrostatic capacitance of the acoustic surface wave device can be decreased, whereby the impedance of the device can be increased. Nevertheless, it has been observed that the following problems are encountered by the conventional approach of electrode division as shown in FIGS. 1 to 5. More specifically, the FIG. 1 device comprises a connecting electrode finger 4 of an increased width for connecting two divided electrode portions 10 and 20. Therefore, the impedance at the electrode finger 4 causes a phase difference in the signal to be applied to the divided electrode portions 10 and 20 and thus a phase difference in the acoustic surface wave energy excited by the respective divided electrode portions 10 and 20. In addition, the acoustic surface wave is not excited within the region of the connecting electrode finger 4 per se, which causes discontinuity of the intensity of the acoustic surface wave energy between the two divided electrode portions 10 and 20.
The acoustic surface wave devices shown in FIGS. 2 and 4 both comprise in the dividing region, electrode fingers 5 and 6 which are placed in the intermediate potential and thus in the same potential. Accordingly, the voltage applied between adjacent electrode fingers 5 and 6 is zero, with the result that the acoustic surface wave is not excited in the region between the divided electrode portions 10 and 20. Therefore, again the intensity of the acoustic surface wave energy is discontinuous at the region between the electrode fingers 5 and 6.
In case of the acoustic surface wave devices shown in FIGS. 3 and 5, the voltage developed between the two adjacent electrode fingers 7 and 8 is as large as two times the voltage developed between any other two adjacent electrode fingers and thus the intensity of the acoustic surface wave energy excited between the electrode fingers 7 and 8 is also as large as two times the energy intensity excited between any other two adjacent electrode fingers, with the result that the acoustic surface energy intensity is discontinuous in the dividing region between the electrode fingers 7 and 8.
The U.S. Pat. No. 3,699,364 issued Oct. 17, 1972 to Henry M. Gerard and entitled "ACOUSTIC SURFACE WAVE DEVICE HAVING IMPROVED TRANSDUCER STRUCTURE" discloses an acoustic surface wave device wherein a desired frequency characteristic is attained by changing the overlapping lengthes of the adjacent electrode fingers that are overlapped with each other in accordance with a weighting function determined by an impulse response obtained by inverse Fourier transformation of the above described frequency characteristic. Such weighted electrode pattern of the electrode fingers is advantageously utilized in an increased attenuation of the side lobe of the frequency characteristic, a non-symmetrical frequency characteristic with given frequency positions attenuated and the like. Nevertheless, the weighted electrode pattern of the electrode fingers increases discontinuity of the acoustic surface wave energy intensity at the dividing region as compared with the ordinary electrode pattern of the electrode fingers, if the electrode division is employed in such weighted electrode pattern type acoustic surface wave device.
The above discussed discontinuity of the acoustic surface wave energy intensity reduces the freedom in designing of a transducer for an acoustic surface wave device. More specifically, the above described discontinuity of the acoustic surface wave energy intensity makes impossible designing of an acoustic surface wave device having some type of frequency characteristic. The present invention is directed to an improvement in an acoustic surface wave device as shown in FIGS. 3 and 5 having two adjacent electrode fingers at the ends close to each other of the divided comb shaped electrodes, wherein the freedom of designing of the device is increased.